Etching zinc plates



Patented Mar. 19, 1935 UNITED STATES PATENT OFFICE ETCHING ZINC PLATESNo Drawing. Application December 31, 1932,

- Serial No. 649,733

'12 Claims.

The present invention relates to etching engravers zinc plates and tonovel etching acids comprising nitric acid in which a metal and anorganic hydroxy compound is dissolved.

I have shown in my co-pending application, Ser. No. 649,731 forEngravers etching acid of increased efficiency, filed on even dateherewith, that the addition of small'amounts of copper, silver ,orplatinum todilute nitric acid as commonly used for etching zinc platesconsiderably increases the emciency of such an etching bath. I have alsoshown in co-pending application, Ser. No. 649,732 for Etching ofengravers zinc I plates", filed on even date herewith, that the foretching zinc plates of small amounts of certain organic hydroxycompounds produces an etching bath which produces much smoother and moreuniform plates. v

I have now found that the presence in a dilute nitric acid etching bathof a small amount of an organic hydroxy compound is entirely compatiblewith the presence therein of a small amount of a .metallic additionagent, such as copper, silver or platinum and that a bath which containsboth types of addition agents shows the improvements due to both andthat their effect seems to be enhanced to a remarkable degree.- By theuseof an etching bath containing both types of addition agents I amenabled to produce smoother, cleaner. etchings with an increasedefliciency over the use of a straight nitric acid etching bath or a bathcong one only of these types of addition 881115.

in the photoengraving art, as at present generally practiced, theetching process may be described essentially as follows: A highlypolished mnc plate is coated with a thin layer of an acid resistingenamel, such as glue or shellac, sensitized to light with ammoniumchromate, or with I a layer of albumin similarly sensitized. The zincplate thus sensitized is placed back of a glass plate bearing anopaque-translucent mirror-image-negative film of the picture or designto be 41 reproduced, and exposed to a strong light.

re the light strikes the sensitized zinc surface the enamel or albuminis fixed to the surface of the zinc, developing on the zinc amirror-ime-positive of the design being reproduced. In

the case of the enamel coating, the unflxed portions of the layer arewashed off with water and addition to dilute nitric acid as commonlyusedthe zinc plate is ready for etching. Inthe case' with water. The inkremaining on the plate is dusted with an acid resisting resin and thelatter burned in by heating the plate to the resin's melting point,after which the plate is ready for etching.

The zinc plates to be etched thus have'those parts which are to printblack covered with an acid resisting coating and the other partsexposed. Those parts of the plate which are protected will hereinafterbe referred to as the tops, 10 and the exposed parts as bottoms.

The etching process itself consists essentially of exposing the plateprepared as described to a nitric acid solution, taking a suitableprocedure to prevent the acid attacking the edges of the tops. Thisoperation is commonly performed by placing the plate vertically in aclosed tank and splashing or throwing the acid solution against theplate by means of a paddle arrangement. It is the general practice touse an etching bath containing approximately 10% HNOa. 1

When the etching of the zinc plate has proceeded sufliciently far thatthe bottoms have a noticeable depth, the plate is rinsed, dried and theexposed edges protected against attack by brushing a bank of acidresisting resin against them and fusing this in place by heating theplate to the required temperature. The etching is continued until theedges are again in danger of being undercut, whereupon they are again 30banked with the acid resisting resin. This procedure is continued untilthe desired depth is obtained which, of course, varies with the type ofreproduction, area of the bottoms and so on.

In this procedure it is essential that the bottoms of the plate be quitesmooth. Otherwise the irregularities in the surface will catch the acidresisting resin and in the course of the operation build up still largerirregularities, whichwill interfere with printing from the plate. Thiscoating of resin on the bottoms slows up the operation veryconsiderably. Even on very smooth bottoms there is some such coating,and this must bev removed before the actual solution of the zincunderneath can proceed. However, if the acid removes the resin tooeasily, it will naturally attack the banks on the edges and lead toundercutting before appreciable depth can be obtained.

It is equally essential that there be no undercutting of the edges.vThis is especially true of the work in newspaper plants where papermattes are made from the engravings for the casting of cylindrical type.

The acid etching bath is used until its action becomes unsatisfactorilyslow, as usually evidence by its inability to remove the thin film ofresin from the bottoms. This point is commonly reached when the nitricacid concentration has fallen to approximately 5%. It is well known thataddition of concentrated acid to a spent bath does not revivify itappreciably; hence, about one-half of the nitric acid required by theindustry must be thrown away.

The presence in an engravers nitric acid of small amounts of copper,silver or platinum increases the eiliciency of the bath. By this I meanthe amount of zinc which a given quantity of nitric acid at a specifiedconcentration will dissolve at or above a fixed rate, the rate at whichit is general practice to discard the bath. A liter of straight 10%nitric acid kept at about 40 C. will dissolve about 17 grams of zinc,after which its rate of dissolving zinc is so slow that it cannot beused further for etching zinc plates and is commonly discarded; itsemciency is rated at 17 grams per liter.

The presence in the engravers nitric acid of small amounts, for instancefrom 0.1 to about 2 parts of Cu, Ag or Pt per 100 parts of HNOs,increases this efficiency. For obvious practical reasons I prefer theaddition of copper, though from a purely experimental standpoint silverand platinum produce substantially the same effects as copper.

The addition of small amounts of organic hydroxy compounds to engraversnitric acid produces a bath which in the first place promotes theformation of a very smooth etched surface, very appreciably smootherthan that given by nitric acid alone, thus materially reducing theamount of acid resisting resin which can adhere to the bottoms andpractically eliminating the tendency to build up irregularities on thebottoms. In the second place, nitric acid containing a small amount ofthe organic hydroxy compound removes the thin film of the resinremaining on the bottom when the edges are banked with such resin forprotection from undercutting much more readily and quickly than straightnitric acid. These advantages are also present when the etching bathscontains copper besides the organic hydroxy compound.

The organic hydroxy compounds which I have found to possess the propertyof producing the improved etchings as described above are water solublecompounds which contain in their molecule an alcoholic hydroxygroup,-OH, and which contain another negative group in their molecule.For purposes of the present invention the OH group contained in aromatichydroxy compounds, such as phenol, cresol, etc. is considered to be ofan alcoholic nature. Representative compounds which satisfy the abovedefinition and which have been found to be useful in my invention arefor instance: aliphatic hydroxy carboxylic acids such as citric acid,tartaric acid,- lactic acid, mucic acid; aromatic hydroxy carboxylicacids such as salicylic acid; aliphatic polyhydroxy compounds such asethylene glycol and glycerol; aromatic hydroxy compounds such as phenol,cresol, resorcinol, and their sulfonic acids, saccharids such asglucose, dextrose and sucrose, etc.

Representative negative groups which impart to organic hydroxy compoundsthe property of improving the etching action of nitric acid upon zincplates are for instance the benzene nucleus, the carboxylic,COOH, group,the hydroxyl group itself, the sulfonic acid group, etc.

I have found that straight carboxylic acids. such as acetic acid inwhich the hydroxy group is not of an alcoholic nature do not have theeffect upon an etching bath, as shown by the above alcoholic hydroxycompounds containing a negative group in their molecules; neither isthis effect shown in any substantial degree by straight alcohols.

The amount of hydroxy compound to be added to the etching bath to obtainthe improved plates according to-"my invention is not very critical; it.

will depend to a certain extent upon the specific compound added andupon the nature and character of the zinc plate, type of design to bereproduced, etc. In general the amount should not weight per parts ofHNOa.

Optimum Maximum Citric acid 0. 86 i. 67 T 4.67 3.5

. Amounts of less than 0.05 per 100 I-INO: are generally not desired.

The metallic and hydroxy addition agents can be incorporated in theetching bath at the time of use or I can also conveniently dissolve themetal or its salt and the hydroxy compound in commercial 40 B. nitricacid. This can be shipped without need of particular precautions and allthe engraver has to do is to dilute this concentrated acid to thedesired strength. In preparing these dualaction etching baths I can ingeneral use more copper than in the absence of the hydroxy compound,whereby I obtain still greater efliciency of the the bath without losingthe benefit of a smooth, clean etch.

The amounts of the addition agents in respect to each other'areotherwise not much different from the amounts I can use in bathscontaining only one type of addition agent.

I have for instance prepared a very emcient etching bath consisting of a10% nitric acid in which copper sulfate was dissolved in an amountcorresponding to 0.167 parts Cu per 100 parts HNO: and to which about0.85 parts citric acid per 100 parts HINO: was added. The zinc emciencyof this bath was 21.4 grams per liter. It produced very clean, brightand smooth etchings.

The efliciency of a similarly prepared 10% nitric acid etching bathcontaining 0.33 Cu and 1.67 parts tartaric acid per 1001-1170: was 25.2grams per liter and it produced exceedingly smooth and bright etchings.

A commercial newspaper photoengravingplant was run for 3 weeks using anetching bath of 10% nitric acid containing 0.167 parts Cu and 0.85 partscitric acid per 100. parts HNOa. Approximately 67,000 sq. in. of surfacewere etched with this bath and 1300 lbs. of 40 B. nitric acid consumedduring the time of the test.

Under identical conditions this same plant required for the productionof 22,000 sq. in. of etched surface 580 lbs. of 40 B. nitric acidcontaining no addition agents; and the plates produced with my noveletching bath were much smoother and brighter and could easily bedistinguished by visual examination'from those produced without theaddition agents.

I claim:

1. An engravers etching bath for zinc plates comprising nitric acid anddissolved therein a small amount of a metal of the group of metalsconsisting of copper, silver and platinum, and a water soluble, organic,alcoholic hydroxy compslund containing a negative group in its molec e.

2. An engraver's etching bath for zinc plates comprising nitric acid anddissolved therein a small amount or copper and a small amount of anorganic hydroxy compound selected from the group consisting oihydroxy-carboxylic acids, aromatic hydroxy compounds and their sulionicacids, aliphatic poly-hydroxy compounds and saccharids.

3. An engravers etching bath for zinc plates comprising nitric acid anddissolved therein from about 0.1 to 2 parts of copper per 100 parts HNO:and from about 0.05 to 3.5 parts per 100 parts END; of an organichydroxy compound selected from the group consisting ofhydroxy-carhoxylic acids, aromatic hydroxy compounds and their sulfonicacids, aliphatic poly-hydroxy compounds and saccharids.

4. An engravers etching bath for zinc plates comprlsingnitric acid anddissolved therein a small amount of copper and a small amount of analiphatic hydroxy carboxylic acid.

5. An engravers etching bath for zinc plates comprising nitric acid anddissolved therein a small amount of copper and a small amount of citricacid.

6. A process of etching a zinc plate which .comprises treating theprepared plate with an etching bath comprising nitric acid and dissolvedtherein a small amount of a metal of the group of metals consisting ofcopper, silver and platinum, and a water soluble, organic alcoholichydroxy compound containing a negative group in its molecule. I

7. A process of etching a zinc plate which comprises treating theprepared plate with an etching oath comprising nitric acid and dissolvedtherein a small amount of copper and a small amount of an organichydroxy compound selected from the group consisting ofhydroxy-carboxylic acids, aromatic hydroxy compounds and their sulionicacids, aliphatic poly-hydroxy compounds and saccharids.

8. "A process of etching-a zinc plate which comprises treating theprepared plate with an etching bath comprising nitric acid and dissolvedtherein from about 0.1 to 2 parts of copper per 100 parts HNO: and fromabout 0.05 to 3.5 parts per 100 parts HNO: of an organic hydroxycompound selected from the group consisting of hydroxy-carboxylic acids,aromatic hydroxy compounds, and their sulionic acids, aliphaticpolyhydroxy compounds and saccharids. I

9. A process of etching a zinc plate which comprises treating theprepared plate with an etching bath comprising nitric acid and dissolvedtherein a small amount of copper and a small amount of an aliphatichydroxy-carboxylic acid.

10. A process of etching a zinc plate which comprises treating theprepared plate with an etching bath comprising nitric acid and dissolvedtherein a small amount of copper and a small amount of citric acid.

11. An engravers etching bath for zinc plates comprising nitric acid anddissolved therein a small amount of copper and a small amount oftartaric acid.

12. A process of etching a zinc plate which comprises treating theprepared plate with an etching bath comprising nitric acid and dissolvedtherein a small amount of copper and a small amount of tartaric acid.

ERNEST R. BOLLER.

